The present invention relates to a method for detecting a focussing condition of an objective lens with respect to an object such as a record medium on which a light spot has to be focussed by said objective lens and to an apparatus for carrying out such a focus detecting method.
Such focus detecting method and apparatus are advantageously applied to an apparatus in which a scanning light spot is projected by an objective lens onto one or more information tracks recorded spirally or concentrically on a disc-shaped record medium to read out an information signal recorded along the track.
In an apparatus for reproducing or picking-up an information signal from the above mentioned record medium, the record medium is usually called as a video disc in which encoded video and audio signals are recorded as optical information such as optical transmissivity, reflection and phase properties. While the video disc is rotated at a high speed such as thirty revolutions per second, i.e. 1,800 rpm, a laser beam emitted from a laser light source such as a helium-neon gas laser is focussed on the tracks of the disc as a light spot and the optical information is read out. One of important properties of such a record medium is a very high density of recorded information and thus a width of the information track is very narrow and a space between successive tracks is also very narrow. In a typical video disc described in, for instance, Philips Technical Review, Vol. 33, 1973, No. 7, a pitch of the tracks amounts only to 2 .mu.m. Therefore the diameter of light spot should be correspondingly small such as 1 to 2 .mu.m. In order to pick-up correctly the recorded information from such tracks having very narrow width and pitch, an error in a distance between the objective lens and the tracks, i.e. a focussing error should be reduced as little as possible to make a spot diameter as small as possible.
To this end, the apparatus is provided with a focussing control system in which an amount and a direction of a de-focussed condition of the objective lens with respect to the disc surface are detected to produce a focussing error signal and the objective lens is moved in a direction of the optical axis of objective lens in accordance with the detected focussing error signal.
FIG. 1 is a schematic view illustrating a known focus detection system in an optical pick-up apparatus. A light source 1 comprises a laser and emits light which is linearly polarized in a plane of the drawing of FIG. 1. The light is collimated by a collimator lens 2 into a parallel light beam which is then transmitted through a polarizing prism 3 and a quarter-wavelength plate 4. The light beam is further focussed by an objective lens 5 as a light spot on a disc 6 having one or more information tracks 6a of crenellated pit construction. Then, the light is reflected by the information track and impinges upon the polarizing prims 3 by means of the objective lens 5 and the quarter-wavelength plate 4. The light impinging on the prism 3 is polarized in a direction perpendicular to the plane of the drawing, because it has transmitted through the quarter-wavelength plate 4 twice and thus, is now reflected by the polarizing prism 3. The light flux reflected by the polarizing prism 3 is converged by a condenser lens 7 and a cylindrical lens 8. Since the cylindrical lens 8 has a focussing power only in one direction, the shape of the focussed beam formed by the condenser lens 7 and the cylindrical lens 8 varies as shown in FIG. 1 with respect to an in-focussed condition in mutually orthogonal directions, when the disc 6 moves up and down. In the known apparatus, this variation in shape is detected by a light detector (not shown) divided into four sections and arranged at a focal plane of the lens system 7, 8 to produce a focussing error signal. The focussing error signal thus detected is supplied to a focussing mechanism such as a moving coil mechanism to move the objective lens 5 in its axial direction.
In the known focus detecting system, since a relatively long optical path is required to focus the light beam after being reflected by the polarizing prism 3, there is a drawback that an optical system is liable to be large in size. Further, since the light detector having the four sections must be arranged precisely in three axial directions, i.e. in the optical axis direction and in two orthogonal directions perpendicular to the optical axis, the adjustment in positioning the light detector is quite critical and requires a time-consuming work. Moreover, since a dynamic range in which the accurate focussing error signal can be obtained due to the deformation of the focussed beam is relatively small, any focussing error signal could not be produced if the disc deviates from a given position only by a relatively small distance.
There has been proposed a method and an apparatus which can obviate the above mentioned drawbacks and can detect a focussing error signal of an objective lens with respect to an object onto which a light spot is to be focussed, which method and apparatus have an extremely high sensitivity for focus detection. The method and apparatus of this type are described in Japanese Patent Application No. 54-74,943 filed on June 25, 1979 corresponding to continuation-in-part patent application Ser. No. 195,075 filed on Oct. 8, 1980.
FIG. 2 is a schematic view illustrating an embodiment of the optical pick-up apparatus thereby proposed. In this apparatus, a linearly polarized light beam emitted from a laser light source 11 is collimated into a parallel light beam by a collimator lens 12 and passes through a polarizing prism 13 and a quarter-wavelength plate 14. Then, the parallel light beam impinges upon an objective lens 15 and is focussed on an information track of a disc 16 as a small light spot. The light beam reflected by the disc 16 is optically modulated in accordance with information recorded in the track and is reflected by the polarizing prism 13. The construction and operation of the optical system so far explained are entirely same as those of the known optical system shown in FIG. 1. The light flux reflected by the polarization prism 13 impinges upon a detection prism 17 having a reflection surface 18 and the light flux reflected by this surface 18 is received by a light detector 19. The reflection surface 18 is so arranged with respect to the incident light that under an in-focussed condition it makes a given angle wih respect to the incident light (parallel light flux) which angle is equal to a critical angle or slightly smaller or greater than the critical angle. Now, for the time being, it is assumed that the reflection surface 18 is set at the critical angle. In the in-focussed condition, the whole light flux reflected by the polarizing prism 13 is totally reflected by the reflection surface 18. In practice, a small amount of light is transmitted into a direction n shown in FIG. 2 due to incompleteness of a surface condition of the reflection surface 18. However, such a small amount of transmitted light may be ignored. If the disc 16 deviates from the in-focussed condition in a direction a in FIG. 2 and a distance between the objective lens 15 and the disc 16 is shortened, the light reflected by the polarizing prism 13 is no longer the parallel beam, but changes into a diverging light beam including extreme light rays ai.sub.1 and ai.sub.2. On the contrary, if the disc 16 deviates in the opposite direction b, the parallel light beam is changed into a converging light beam including extreme light rays bi.sub.1 and bi.sub.2. As can be seen in FIG. 2, light rays from an incident optical axis OP.sub.i to the extreme light ray ai.sub.1 have incident angles smaller than the critical angle and thus, are transmitted through the reflection surface 18 at least partially as illustrated by at.sub.1 (the reflected light being shown by ar.sub.1). Contrary to this, light rays between the optical axis OP.sub.i and the extreme light ray ai.sub.2 have incident angles larger than the critical angle and thus are totally reflected by the surface 18 as shown by ar.sub.2. In case of deviation of the disc 16 in the direction b, the above relation becomes inversed, and light rays below a plane which includes the incident optical axis OP.sub.i and is perpendicular to the plane of the drawing of FIG. 2, i.e. a plane of incidence, are totally reflected by the reflection surface 18 as denoted by br.sub.1, and light rays above said plane are at least partially transmitted through the reflection surface 18 as depicted by bt.sub.2. As explained above, if the disc 16 deviates from the in-focussed position, the incident angles of the light rays impinging upon the reflection surface 18 vary in a continuous manner about the critical angle except for the center light ray passing along the optical axis OP.sub.i. Therefore, when the disc 16 deviates from the in-focussed position either in the direction a or b, the intensity of the light reflected by the reflection surface 18 varies abruptly near the critical angle in accordance with the above mentioned variation in the incident angles. In this case, senses of the variations of the light intensities on both sides of said plane perpendicular to the incident plane and including the incident optical axis OP.sub.i vary in mutually opposite manner. On the contrary, in the in-focussed condition, the light flux impinging upon the detection prism 17 is totally reflected by the reflection surface 18 and thus, the uniform light flux impinges upon the light detector 19. The light detector 19 is so constructed that the lower and upper light fluxes with respect to said plane are separately received by separate regions 19A and 19B, respectively. That is to say, the light detector 19 is divided along a plane which is perpendicular to the incident plane and includes an optical axis OP.sub.r of reflected light.
In FIG. 2, if the disc 16 deviates in the direction a, the light rays of the lower half of the incident light flux have incident angles smaller than the critical angle. Therefore, at least a part of the lower half light flux is transmitted through the reflection surface 18 and the amount of light impinging upon the light receiving region 19A is decreased. While the upper half of the incident light flux has the incident angles larger than the critical angle and thus, is totally reflected by the surface 18. Therefore, the amount of light impinging upon the light receiving region 19B is not changed. On the contrary, if the disc 16 deviates in the direction b, the amount of light impinging upon the region 19B is decreased, but the amount of light impinging upon the region 19A is not changed. In this manner, the output signals from the regions 19A and 19B vary in an opposite manner. A focussing error signal can be obtained at an output 21 of a differential amplifier 20 as a difference signal of these signals from the regions 19A and 19B.
The reflection surface 18 may be set at an angle slightly smaller than the critical angle. In such a case when the disc 16 deviates in the direction a, the amount of light impinging upon the region 19B is first increased and then becomes constant and the amount of light impinging upon the region 19A is decreased abruptly. Whereas, if the disc 16 deviates in the direction b, the amount of light impinging upon the region 19A is first increased and then becomes constant, while the amount of light impinging upon the region 19B is decreased abruptly.
In this manner by detecting a difference in output signals from the light receiving regions 19A and 19B, it is possible to obtain the focussing error signal having an amplitude which is proportional to an amount of the deviation from the in-focussed condition and a polarity which represents a direction of the deviation with respect to the in-focussed condition. The focussing error signal thus obtained is used to effect a focussing control for driving the objective lens 15 in the direction of its optical axis. Further, it is possible to derive an information signal corresponding to the pit information recorded in the information track at an output 23 of an adder 22 which produces a sum signal of the output signals from the regions 19A and 19B. Further, in the in-focussed condition, since the light is scarcely transmitted through the reflection surface 18, a loss of light is very small and in the defocussed condition the half of light flux with respect to the central light ray is totally reflected, but an amount of the other half of light flux reflected by the surface 18 is decreased to a great extent, the difference in the amount of light impinging upon the regions 19A and 19B becomes great. Therefore, the very accurate focus detection can be effected with a higher sensitivity as compared with the known apparatus shown in FIG. 1.
Various experiments and tests have discovered that the sensitivity of the focus detecting apparatus illustrated in FIG. 2 is not sufficiently high and thus could hardly be applied to the focussing control system in the video or audio disc player which requires an extraordinary high accuracy and particularly the stable focussing error signal could not be obtained precisely due to cross-talk of a track signal. In the above mentioned patent application, it has been further proposed to use an elongated detection prism 17' shown in FIG. 3. In this elongated prism 17', the light beam reflected by the polarizing prism 13 is reflected several times between parallel reflection surfaces 18' of the prism 17'. When a reflection ratio of a single total reflection is 1/T, after reflection of N times, the reflection ratio becomes 1/T.sup.N and thus, the sensitivity becomes high exponentially. However, the size of the detection prism 17' is liable to be large and thus, it is impossible to obtain a compact optical system. Further, since the opposite reflection surfaces 18' must be accurately in parallel with each other, the detection prism 17' could not be manufactured easily and becomes expensive.